1. Field of the Invention
Electrolytic capacitors and, more particularly, multiple anode stacked capacitor constructions comprising folded anode assemblies and/or conjoined capacitor assemblies for use with electrolytic capacitors are disclosed, as well as electrolytic capacitors comprising the anode assemblies and/or capacitor configurations.
2. Background of the Invention
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors typically consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. While aluminum is generally used for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. Flat constructions for aluminum electrolytic capacitors are known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween and connections between the various anode and cathode layers made via tabs on each individual electrode layer.
The need for high voltage, high energy density capacitors is most pronounced when employed in implantable cardiac defibrillators. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number and volume of the anodes. For example, a multiple anode flat, stacked capacitor configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. In order to achieve higher energy densities, three, four and five anodes can be stacked per layer. Maximization of the anode volume may also be accomplished by etching to achieve more effective anode surface area, and making the relative size of the anode plates larger with respect to the cathode plates.
Current multiple anode flat, stack capacitor configurations comprise on the order of 78 separate components (including anodes, cathodes and separator papers) which leads to substantial manufacturing time and cost, as well as labor intensive assembly processes. In addition, a significant portion (about 13%) of the capacitor volume is used to align the various components of the stack configurations (for example, through alignment holes) and to make the electric connections between the various parts (e.g., by tabs). This reduces the energy density of the capacitor and also increases the volume and mass of the ICD. There is, therefore, a need for improved methods and configurations that reduce the cost and time associated with flat capacitor manufacturing and assembly, while increasing the energy density and reducing the volume and mass of the capacitor configurations.